The operating temperature within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-based superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10-25 times atmospheric pressure, and adiabatically heated to about 800°-1250° F. (425°-675° C.) in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of about 3000° F. (1650° C.). These hot gases pass through the turbine, where rotating turbine wheels extract energy to drive the fan and compressor of the engine. The gases then pass into the exhaust system, where the gases supply thrust to propel the aircraft. To improve the efficiency of operation of the aircraft engine, combustion temperatures have been raised. Of course, as the combustion temperatures are raised, steps must be taken to prevent thermal degradation of the materials forming the flow path for these hot gases of combustion.
An aircraft gas turbine engine has a turbine to drive its compressor. In many designs, the turbine is subdivided into a high pressure turbine (HPT) and a low pressure turbine (LPT). The HPT is located just behind the combustor in the engine layout and experiences the highest temperature and pressure levels, nominally 2400° F. (1315° C.) and 300 psia respectively, developed in the engine. The HPT also operates at very high speeds (10,000 RPM for large turbofans, 50,000 for small helicopter engines). In order to meet life requirements at these levels of temperature and pressure, the HPT today is always cooled with supplemental air cooling techniques and constructed from advanced alloys.
While a straight turbojet engine will usually have only one turbine, most engines today are of the turbofan, either of the high bypass or low bypass type, or turboprop type and require one or two additional LPT turbines to drive a fan or a gearbox. Since substantial pressure drop occurs across the HPT as the HPT extracts energy from the hot fluid stream, the LPT operates with a much less energetic fluid and will usually require several stages (usually up to six) to extract additional energy from the stream.
Components formed from iron, nickel and cobalt-based superalloys cannot withstand long service exposures if located in certain sections of a gas turbine engine, where temperature is elevated, such as the LPT and HPT sections. A common solution is to provide such components with an environmental coating that inhibits high temperature oxidation and hot corrosion. Coating materials that have found wide use for this superalloy generally include diffusion aluminide coatings. These coatings are generally formed by such methods as diffusing into a substrate matrix aluminum deposited by chemical vapor deposition (CVD) or slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase aluminide (VPA) deposition. In the high-pressure stages, aluminum-containing coatings are employed that form stable alumina film. In the low-pressure stages, chromium-containing coatings are favored.
Component surfaces may also include metallic heat rejection coatings, such as platinum. These heat rejection coatings assist in reducing component temperature by effectively reflecting the radiative energy away from the component surface. Accordingly, it is highly desirable to apply these heat rejection coatings to similarly exposed surfaces. However, this is not possible for certain metal alloy parts, such as HPT and LPT components, which may be regularly exposed to temperatures exceeding about 1450° F. (788° C.). In this temperature range, the heat rejection coating interdiffuses with the underlying metallic component surface, or substrate, which is also a metal. In essence, a portion of the heat rejection coating material migrates into the component substrate material as elements of the substrate migrate in the opposite direction through the heat rejection coating forming oxides on its surface. This interdiffusion causes the reflective heat rejection surface to become a radiation absorber, losing its ability to reflect radiative energy, resulting in a reduction of its ability to decrease component surface temperature, thereby decreasing the service life of the component.
A diffusion aluminide coating generally has two distinct zones, the outermost of which is an additive layer containing an environmentally resistant intermetallic generally represented by MAl, where M is iron, nickel, or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradient and changes in elemental solubility in the local regions of the substrate. During high temperature exposure in air, the additive layer forms a protective aluminum oxide (alumina) scale or layer that further inhibits oxidation of the underlying substrate. The oxide layer formed over the diffusion aluminide provides a diffusion barrier that inhibits interdiffusion of the heat rejection coating with the substrate.
The prior art solutions for applying diffusion aluminide coatings including VPA and CVD are complicated, have environmental drawbacks, and are inherently costly. What is needed is a less costly approach to applying diffusion aluminide coatings that is more environmentally friendly. These diffusion aluminide coatings may be used as a low-cost oxidation protection barriers and diffusion barrier to prevent interdiffusion of heat rejection coatings.